How many items should I migrate simultaneously?

In a bandwidth-constrained environment, it is important to specify the number of items to migrate simultaneously so that the network does not become overutilized. An overutilized network may cause timeouts during migration, which result in items being skipped.

Also see our article: Size your migration appropriately.

Refer to KB004627, for instructions on how to set the value for the number of concurrent mailboxes to migrate.